Continuous Digester is a tubular reactor in which wood chips reacts with an aqueous solution of sodium hydroxide and sodium sulfide, known as White liquor, to remove the lignin content from the cellulose fibers. The product of the digesting process is cellulose fibers, called pulp, which is used to make paper products. Most of the Digesters consist of three basic zones as shown in FIG. 1 in the continuous digester process 100, an impregnation zone 130, one or more cooking zones 140, and a wash zone 150. The white liquor 125 penetrates and diffuses into the wood chips received from the wood chip bin 110 through the impregnation vessel 120, as they flow through the impregnation zone. The white liquor and chips are then heated to reaction temperatures and the lignin is removed as pulp moved down though cooking zone, where the majority of delignification reactions occur. The wash zone is the end of digester where a countercurrent flow of free liquor washes the degraded products from pulp. The wash zone also cools off the pulp that is discharge through the pulp blow/discharge line 170 so as to quench the reaction and reduce the damage to cellulose fibers from continued reaction. Kappa number (Kappa) is a measure of the residual lignin in the pulp and is a direct indicator of pulp quality. Kappa number is defined by Technical Association of the Pulp and Paper Industries (TAPPI) in standard T-236. The known relationship between the Kappa number and lignin content is that the percent lignin in the pulp equals 0.147 times the Kappa number.
Kappa number which is a measure of delignification is usually measured at Blow line either by an On-line Analyzer or measured in a laboratory. The measured Kappa is then used for feedback control which manipulates H-Factor target. H-factor regulates the lower cooking zone temperature for a given production rate, provided the effective alkali to wood ratio is unchanged.
There is a long time delay between the measured Kappa at Blow line and temperature change in lower cooking zone (manipulated variable). Because of the slow process, the existing kappa feedback control cannot correct the fast variations in Kappa number. Therefore, goal of existing kappa feedback control is to take care of the slow variations in wood composition and other cooking conditions, and maintain the Kappa number at its target.
There are several other model-based control methods for maintaining the Kappa number in continuous digesters, in which a physics based model of the continuous digester is used to determine best operating conditions i.e. H factor, alkali wood ration etc. to maintain the Kappa number under specified production conditions. The major limitation with such approaches, however, is that the process models are usually nonlinear and consist of several mathematical equations. Such models require a high level of expertise to calibrate and tune and are often practically infeasible to be implemented by an average process or control engineer, thus rendering such control applications expensive and difficult to maintain. Several of these models are based on first principle kinetic models, hence it has limitations on the practical implementation.
Another important parameter is the chip level in the digester. The chip level is the level of the total contents of the digester at any given time as measured in the top section of the digester. Normally, maintaining a steady chip level of 50-60% results in a stable cooking process in the digester and consequently a consistent Kappa number. Frequent variations in the digester level result in disturbances to the cooking process and hence an inconsistent pulp quality i.e. varying Kappa. A high chip level in the digester results in under-cooking of the pulp and hence results in an increased Kappa i.e. the resulting pulp contents higher lignin content than desired. A low chip level in the digester results in over-cooking of the pulp and yields a pulp containing less lignin content than desired. One way to address the effect of chip level variations on pulp quality or Kappa is to vary the cooking conditions (H Factor or cooking zone temperature) in accordance with the level variations. But such variations in cooking conditions result in a non-optimal and inconsistent operation of the digester. Hence, it is important to control the chip level in the digester.
Conventional methods for chip level control rely on adjusting the speed of the outlet device or bottom scraper of the digester and/or the blow or discharge flow rate. The bottom scraper scrapes/pushes out the pulp from the bottom of the digester. The blow or discharge flow rate is the rate of flow of pulp in the blow or discharge line from the digester. While large variations in scraper speed result in variations in pulp consistency (liquor content in the pulp), variations in blow/discharge flow affect the actual production rate from the digester resulting in variations in downstream processes such as pulp washing. Furthermore, the behavior of the chip level in the digester is nonlinear and may not be controlled efficiently using a linear controller. Thus, to achieve a stable chip level and cooking conditions in the digester while also ensuring stable production rates, it is necessary to employ a multivariable optimal control approach that optimally adjusts all relevant process variables taking into account the multivariable dynamics that exists in the continuous digester process and also addresses the nonlinearity in the dynamics.
Due to the complex nature of delignification process and significant residence times in various zones of the continuous digester, it is difficult to maintain the quality variables in the digester. Furthermore, the Kappa number of pulp produced from digester can only be measured physically in the blow line, i.e. the current measured value of Kappa is the result of past process input parameters. Any control based on such a measurement would result in a “reactive” action i.e. the controller would act only after the effect of the current process conditions has been realized at the outlet. Hence, in order to maintain a steady process with minimum variation in the quality of pulp, it is necessary to get an assessment of the Kappa in the cooking zone itself so that any deviation of this cooking zone Kappa can then be immediately addressed by optimally varying the input variables and by also considering the effect that would be seen at the outlet i.e. the blow line.
Therefore, there is a need for a system that considers the nonlinear multivariable effects in the continuous cooking process, predicts quality variables at such locations in the digester where measurements do not exist and then optimally control such variables by varying the input variables in an optimal fashion such that chemical losses are also minimized. The principal object of the invention is therefore to meet the above need by a method and system for inferential predictive optimal control of a continuous digester.